Arrival of Buddhism in Southeast Asia
Buddhism reached Sri Lanka about the middle of the 3rd century B.C. From there and from India, some centuries later, it spread to Southeast Asia, reaching Cambodia, Sumatra and Java by the A.D. 3rd century and Burma by at least by the A.D. 5th century. It also took hold to a lesser extent in Malaysia and Borneo and remained strong in there and in Indonesia until the massive conversion to Islam in the 15th century.
Buddhism may have arrived earlier. According to Buddhist tradition, Emperor Ashoka sent missionaries to Suvanaphoum (the Golden Land) is the 3rd century B.C. Suvanaphoum was an emerging area of Indian and Chinese culture is thought to have embraed southern Myanmar, Thailand and eastern Cambodia.
Joe Cummings wrote in the Lonely Planet Guide for Thailand: “Theravada Buddhism was flourishing and may have entered the region during India's Ashoka period, in the 3rd or 2nd century BC, when Indian missionaries are said to have been sent to a land called Suvannabhumi (Land of Gold). Suvannabhumi most likely corresponds to a remarkably fertile area stretching from southern Myanmar, across central Thailand, to eastern Cambodia. Two different cities in Thailand's central river basin have long been called Suphanburi (City of Gold) and U Thong (Cradle of Gold). [Source: Joe Cummings, Lonely Planet Guide for Thailand]
Mahayana Buddhism may have been the first form of Buddhism to really take hold in Southeast Asia. It arrived in northern Burma from India and remained there from the 5th century to the 11th century as was the case in India. Buddhist monks from India and China also brought with the knowledge of medicine and science from those cultures. Mahayana Buddhism is believed to have arrived in southern Southeast Asia via the Kingdom of Srivjaya in Indonesia or Funan, where it was practiced in the A.D. fifth century.
In the 8th century the powerful Shri-Vijaya kingdom in Sumatra introduced a mixture of Hinduism and Mahayana to the Khmers in present-day Cambodia. The Khmers were originally Hindus. In the late 12th century, Jayavarman VII made Mahayana Buddhism the state religion. Mahayana was compatible with the form of Hinduism and the god-king concept that existed in Cambodia at that time. It was expressed in Sanskrit. The Khmers converted to Buddhism in the 12th century under king Jayavarman VII (See Javyavarman VII, Cambodia) but continued to acknowledge Hinduism and worship many Hindu gods, particularly Shiva and Vishnu. The Khmers spread Mahayana Buddhism across Southeast Asia until their kingdom collapsed in the 14th century.
Buddhism had all but died out its homeland of India when it arrived in Southeast Asia. It provided a philosophical and oral framework for people that extended from Tibet to Vietnam. Even though Buddhism became the predominate religion, Hinduism and animism and local religions remained alive and fused together in a way that was unique to the region, embracing some Hindu deities and cult practices and absorbing some animist spirits. Many legends that became part of local folklore have both Hindu and Buddhist elements. Sometimes even different elements of the same religion came together in unique ways. The god Hara-Hara, popular in Khmer art, was a combination of Shiva and Vishnu.